Ti-based shape memory alloy article

ABSTRACT

An article of Ti-based shape memory alloy is made of a Ti-based alloy containing, by atomic percentage, 4 to 10 at % Mo, at least one of 3 to 10 at % Sn and 1 to 10 at % Sc, and the balance Ti and unavoidable impurities, or a Ti-based alloy containing 15 to 30 at % Nb, 1 or more at % Sc, and the balance Ti and unavoidable impurities.

This application claims priority to prior Japanese patent application JP 2004-93567, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention relates to a Ni-free shape memory alloy material and, in particular, relates to a Ti-based shape memory alloy article suitable as a biomaterial that is temporarily or permanently used at every possible portion of a human body.

It is well known that Ti—Ni based shape memory alloys exhibit a remarkable shape memory effect and superelasticity following reverse transformation of the martensitic transformation. Shape memory alloy articles made of those alloys are used in the following applications.

For example, in the shape memory effect application, the shape memory alloy articles are used as thermal actuators of air conditioners, microwave ovens, and so on. In the superelasticity application, the shape memory alloy articles are widely used as medical supplies like catheter guide wires, stents, and so on and as daily goods like eye glass frames, bracelets, and so on. However, in view of concerns about occurrence of a problem of metal allergy or cytotoxicity which would be otherwise caused by direct contact between Ni and a human body, the Ti—Ni alloy is coated with a resin or the like in the relevant application.

In recent years, studies have been made on Ti-based alloys free of Ni and made of elements that do not cause metal allergy.

For example, Japanese Unexamined Patent Application Publication (JP-A) S53-123323 (hereinafter referred to as Patent Document 1) discloses a corrosion-resistant titanium alloy containing, by weight, 1 to 2 wt % Ag (silver) in a β-Ti alloy.

Japanese Unexamined Patent Application Publication (JP-A) H01-129941 (hereinafter referred to as Patent Document 2) discloses a cold-working low-strength high-ductility Ti alloy containing 6 wt %≦Mo (molybdenum)≦18 wt %, 0.5 wt %≦Sn≦10 wt %, and the balance Ti.

Further, Japanese Unexamined PatentApplication Publication (JP-A) H04-214830 (Japanese Patent (JP-B) No. 2936754; hereinafter referred to as Patent Document 3) discloses a Ti alloy similar to that in Patent Document 2.

On the other hand, Japanese Unexamined Patent Application Publication (JP-A) S59-56554 (corresponding to Japanese Examined Patent Application Publication (JP-B) S59-35978); hereinafter referred to as Patent Document 4) discloses a shape memory titanium alloy containing 10 to 15 wt % Mo in titanium.

However, in Patent Document 1, although an improvement in corrosion resistance in several kinds of β-Ti alloys is confirmed as specific examples, the corrosion resistance of all β-Ti alloys is not necessarily improved and there is no description at all about springiness.

In Patent Document 2, although a high-workability low-strength alloy is introduced, there is no description about springiness.

In Patent Document 3, an alloy with further improved workability is discussed and whereupon description is given of an alloy containing 0.5 to 6 wt % Sn, which, however, is merely a low-strength alloy with improved workability and does not necessarily have springiness.

Further, in Patent Document 4, although a stress-induced martensite α-stabilized by addition of Al, being an alloy having the shape memory properties, exhibits the shape memory properties, there is no description at all about springiness. Particularly, any of Patent Documents 1 to 4 gives no consideration to adaptability to the human body.

In view of this, for the purpose of providing high-strength Ti-based alloy spring materials having less indication of toxicity and allergenicity and being human body alloys excellent in workability and biocompatibility, the present inventors have proposed a Ti—Mo based alloy spring (JP-A 2004-183014) and a Ti—Sc based shape memory alloy (JP-A 2004-204245).

SUMMARY OF THE INVENTION

It is an object of this invention to further extract compositions and production conditions capable of achieving a shape memory effect and compositions and production conditions capable of achieving superelasticity to thereby provide practical shape memory alloy articles.

According to this invention, there are obtained Ti-based shape memory alloy articles made of alloys having the following compositions.

According to one aspect of the present invnetion, there is provided an article of Ti-based shape memory alloy which comprises a Ti-based alloy having a composition containing 4 to 10 at % Mo, at least one of 3 to 10 at % Sn and 1 to 10 at % Sc, and the balance Ti and unavoidable impurities.

According to another aspect of the present invention an article of Ti-based shape memory alloy which comprises a Ti-based alloy having a composition containing 15 to 30 at % Nb, 1 or more at % Sc, and the balance Ti and unavoidable impurities.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is photographs of metallurgical structures for showing the results of shape memory property evaluation of Ti-5Mo-(3-6)Sn (at %) alloys after a treatment at 1000° C.;

FIG. 2 is a diagram showing a relationship between aging temperature and material hardness of the Ti-5Mo-5Sn alloy;

FIG. 3 is photographs of metallurgical structures for showing the results of property evaluation of Ti-5Mo-(4,5)Sn alloy aging-treatment materials at 600° C. at 5 minutes;

FIG. 4 is a diagram showing the results of shape recovery property evaluation of Ti-6Mo—Sc alloys following a change in Sc concentration;

FIG. 5 is a diagram showing the influence of aging conditions exerted on the material hardness of a Ti—Mo-7Sc alloy;

FIG. 6 is photographs of metallurgical structures for showing the results of property evaluation of Ti-6Sc-(18-24)Nb (at %) alloys after a β-transformation treatment;

FIG. 7 is photographs of metallurgical structures for showing the results of property evaluation of a Ti-6Sc-26Nb alloy aging-treatment material at 600° C. at various times; and

FIG. 8 is a diagram showing the influence of aging conditions exerted on the material hardness of a Ti-6Sc-26Nb alloy.

DESCRIPTION OF THE PREFERRED EMBODIMENT

At first, description will be given of compositions of alloys used in shape memory alloy articles of this invention and the reason for limiting aging conditions.

One shape memory alloy article of this invention comprises a Ti—Mo—(Sn, Sc) based alloy. In this invention, the content of Mo is set to 4 to 10, atomic percent, at %. This is because when it is less than 4 at %, β-stabilization is not sufficiently achieved to thereby cause degradation in workability and properties, while, when it exceeds 10 at %, it becomes difficult to obtain sufficient shape memory properties. The content of Sn or Sc is also set in a range where the effect of addition thereof is significant. Particularly, since Sc is extremely expensive, the content of Sc is limited to the range where the effect thereof is sufficient. Further, the limitation on the aging conditions is for setting conditions that do not affect an increase in hardness.

Further, this alloy can contain not only addition of Sn or Sc outside the limited range, but also several at % of another β-stabilizing element, such as Nb, Hf, and the like or another a-stabilizing element, such as Ag, Al, and the like.

Another shape memory alloy article of this invention comprises a Ti—Nb—Sc based alloy. In this invention, the content of Nb is set to 15 to 30 at %. This is because when it is less than 15 at % or exceeds 30 at %, it becomes difficult to obtain sufficient shape memory properties. The content of Sc is also set in a range where the effect of addition thereof is significant. The limitation on the aging conditions is, as described above, for setting conditions that do not affect an increase in hardness.

Further, this alloy can contain several at % of another β-stabilizing element, such as Mo, Hf, and the like or another α-stabilizing element, such as Sn, Ag, Al, and the like.

According to each of such Ti-based alloys of this invention, there is obtained a shape memory alloy article having a recovery strain of at least 2% and, by performing aging after a heat treatment at or above a β-transformation temperature, there is obtained a shape memory element having a shape memory effect of a 3% or more recovery strain or a 2% or more superelasticity.

The shape memory alloy articles of this invention each have a shape of a plate, a wire rod (linear shape) including a bar, a tube, or the like and can be not only applied to Ni-free biomaterials but also widely applied to eye glass frames, golf supplies, and so on.

In this invention, medical materials and medical instruments represent part or the whole of all metal articles that are buried or brought into contact with human bodies temporarily or over the long term.

In this invention, as shape memory articles each using a shape memory alloy wire rod, there are catheters, guide wires for use in catheters and so on, endoscopes, stents, orthodontic appliances, scrub lines, bone joining wires, clips, and so on, while, as plate-like or bar-like shape memory elements, there are bone plates, artificial sphincters, fixing screws, artificial bones, and so on. This invention, however, is not limited thereto.

On the other hand, as consumer products, they can be used as part or the whole of golf clubs, eye glasss, and timepieces, heat exchangers, heat insulators, vibration suppressors, and so on. With respect to the golf club, they can be used as a shaft, a crown, a face, and so on. With respect to the timepiece, they can be used as the whole thereof, a band piece, a pin, a case, and so on.

This invention will be described in further detail.

In this invention, attention is paid to alloys excellent in biocompatibility and therefore use is made of Ti-based alloys each containing at least two kinds of Mo, Sn, Nb, and Sc with no indication of toxicity or allergenicity while excluding V, Ni, and Co with indication of toxicity.

This invention provides Ti-based alloys each having a crystal structure equivalent to a β-Ti alloy and having a shape memory effect or superelasticity.

Therefore, the Ti-based alloys aimed at by this invention are each a β-Ti alloy or a near β-Ti alloy that is excellent in workability among titanium alloys.

That is, in this invention, as elements contained in the Ti-based alloy, selection is made of Mo, Sn, Nb, and Sc that are elements with no indication of toxicity and excellent in biocompatibility.

(1) Alloy Production

At first, selection was made of alloy compositions that could be β-type or near β-type as shown in Tables 1 and 2 and alloys were produced by argon-arc melting. The melting was performed in an argon atmosphere in an arc melting furnace using a water-cooled copper hearth and non-consumable tungsten electrodes. In order to reduce segregation of alloy components, ingots of the alloys were turned upside down and subjected to melting and freezing six times.

The produced ingots were subjected to a homogenizing treatment in a vacuum atmosphere at 1000° C. for 24 hours and then to furnace cooling. TABLE 1 Shape Memory Properties of Ti—Mo—(Sn, Sc) Alloys Shape Memory Properties β-phased at % Work- Cold Treat- No. Ti Mo Sn Sc ability Working ment Examples 1 bal. 4 5 — Δ ◯ ◯ 2 bal. 5 3 — Δ ◯ ◯ 3 bal. 5 4 — ◯ ◯ ◯ 4 bal. 5 5 — ◯ ◯ ◯ 5 bal. 5 10 — ◯ ◯ ◯ 6 bal. 10 — 5 ◯ ◯ ◯ 7 bal. 4 — 5 ◯ ◯ ◯ 8 bal. 6 — 2 ◯ ◯ ◯ 9 bal. 6 — 7 ◯ ◯ ◯ 10 bal. 6 — 10 ◯ ◯ ◯ 11 bal. 10 — 5 ◯ ◯ ◯ 12 bal. 5 5 1 ◯ ◯ ◯ 13 bal. 6 1 5 ◯ ◯ ◯ Compara- 14 bal. 3 5 — X ◯ ◯ tive 15 bal. 15 5 — ◯ X X Examples 16 bal. 5 2 — ◯ X X 17 bal. 5 15 — ◯ X X 18 bal. 3 — 5 ◯ X X 19 bal. 15 — 5 ◯ X X 20 bal. 5 — 0.5 ◯ X X bal.: balance —: 0 at %

TABLE 2 Shape Memory Properties of Ti—Nb—Sc Alloys Shape Memory Properties at % Work- Cold β-phased No. Ti Nb Sc ability Working Treatment Examples 21 bal. 15 3 Δ ◯ ◯ 22 bal. 18 2 Δ ◯ ◯ 23 bal. 18 1 ◯ ◯ ◯ 24 bal. 18 4 ◯ ◯ ◯ 25 bal. 18 8 ◯ ◯ ◯ 26 bal. 18 10 ◯ ◯ ◯ 27 bal. 18 6 ◯ ◯ ◯ 28 bal. 20 6 ◯ ◯ ◯ 29 bal. 22 6 ◯ ◯ ◯ 30 bal. 24 6 ◯ ◯ ◯ 31 bal. 26 6 ◯ ◯ ◯ 32 bal. 30 6 ◯ ◯ ◯ Compara- 33 bal. 10 6 ◯ X X tive 34 bal. 35 6 ◯ X X Examples 35 bal. 18 0.5 ◯ X X bal.: balance

(2) Sample Production

Subsequently, plates each having a thickness of 2 to 3 mm were cut from the homogenized ingots and then cold-rolled to 0.3 to 0.5 mm. Thereafter, part of each sample was subjected to a heat treatment at 1000° C. for one hour and then quenched in an ice brine so as to be subjected to a β single-phase transformation treatment. Although the cold rolling was carried out, it may be replaced with hot rolling, hot rolling and then cold rolling, or repetition of hot rolling and cold rolling.

(3) Shape Memory Property Evaluation

Property evaluation was conducted on the basis of a simple bending test using the foregoing test pieces. At first, each of the plates having the thickness of 0.3 to 0.5 mm was wound round a cylinder with a radius of 3 to 4 mm. In this event, a strain of about 4 to 5% was applied. Thereafter, unwinding and lighter heating were carried out. The shape memory properties were evaluated according to the total amount of a springback (superelasticity) after the unwinding and a residual strain cancellation (shape memory effect) caused by the heating, wherein a recovery amount of 2% or more was evaluated as good (identified by O in Tables) while a recovery amount of 1 to 2% was evaluated as normal (identified by Δ in Tables).

(4) Aging Property Evaluation

The samples in Tables 1 and 2 having been subjected to the β-transformation treatment at 1000° C. were each subjected to an aging treatment at 100° C. to 700° C. for 5 minutes, thereby evaluating the shape memory properties thereof. The evaluation was conducted like in the foregoing manner. Some of the results are shown in Table 3. TABLE 3 Shape Memory Properties of Ti—(Mo, Nb)—(Sn, Sc) Alloy Shape Memory Properties 100° 150° 200° 400° 500° 600° 700° No. Alloy C. C. C. C. C. C. C. 3 Ti—5Mo—4Sn X X X X ◯ ◯ Δ 4 Ti—5Mo—5Sn X X X X ◯ ◯ Δ 9 Ti—6Mo—7Sc X ◯ ◯ ◯ ◯ ◯ Δ 31 Ti—26Nb—6Sc X ◯ ◯ ◯ ◯ ◯ Δ

(5) Shape Memory Effect and Superelasticity

At first, description will be given with respect to Ti—Mo—Sn based alloys.

FIG. 1 shows the results of property evaluation of the Ti-5Mo-(3-6)Sn (at %) alloys after the treatment at 1000° C., where ε_(d) represents a deformation strain, ε_(s) a residual strain after the springback (superelasticity), and ε_(r) a residual strain after the heating (shape memory effect), respectively. In addition, a standard temperature which will be hereinafter referred to as “S.T.”.

From the results shown in FIG. 1, it is understood that the alloy with lower content of Sn tends to exhibit the shape memory effect, that the superelasticity becomes maximum at 5Sn, and that both properties are degraded at 6Sn.

In the case of Mo, the low content of Mo cannot sufficiently achieve β-stabilization of the alloy and induces generation of an ω phase in the solution treatment or the aging treatment to thereby cause difficulty in workability. On the other hand, the high content of Mo can sufficiently achieve β-stabilization of the alloy, but it becomes difficult to hold a shape memory property of 2% or more.

FIG. 2 shows a relationship between aging temperature and material hardness of the Ti-5Mo-5Sn alloy. From FIG. 2, it is understood that, in the case of holding for 5 minutes (300 seconds) where the aging effect is easily achievable, a significant increase in hardness is observed when the temperature is less than 500° C., thereby causing degradation in properties.

FIG. 3 shows, at (a) and (b), the results of property evaluation of the Ti-5Mo4Sn alloy aging-treatment material and the Ti-5Mo-5Sn alloy aging-treatment material at 600° C. at 5 minutes, respectively. From FIG. 3, it is understood that there is observed significant property change and improvement caused by the aging.

Now, description will be given with respect to Ti—Mo—Sc based alloys.

FIG. 4 shows the shape recovery properties of the Ti-6Mo—Sc alloys following a change in Sc concentration. From FIG. 4, it is understood that the addition effect of Sc is small when Sc is 1 at % or less, while the addition of Sc exceeding 8 at % impedes the properties.

FIG. 5 shows the influence of aging conditions exerted on the material hardness of the Ti—Mo-7Sc alloy. It is understood that, as different from the foregoing case of FIG. 2, an increase in hardness is not observed even at 200° C. and therefore wide condition setting is enabled.

Now, description will be given with respect to Ti—Nb—Sc based alloys.

FIG. 6 shows the results of property evaluation of the Ti-6Sc-(18-24)Nb (at %) alloys after the β-transformation treatment. From FIG. 6, it is understood that the alloy with lower content of Nb tends to exhibit the shape memory effect.

FIG. 7 shows the results of an aging treatment of the Ti-6Sc-26Nb alloy at 600° C. for 7 minutes. From FIG. 7, it is understood that the superelasticity properties are improved by the aging.

FIG. 8 shows the influence of aging conditions exerted on the material hardness of the Ti-6Sc-26Nb alloy. It is understood that, like in the foregoing case of FIG. 2, a significant increase in hardness is observed at less than 500° C.

In the foregoing examples of this invention, the description has been given of the plate-like shape memory alloy articles. However, this invention is not limited to the plate shape but can be modified in various ways depending on the purpose and use. For example, this invention is also applicable to wire rods including bars, tubes, and so on. 

1. An article of Ti-based shape memory alloy comprising a Ti-based alloy which has a composition containing 4 to 10 at % Mo, at least one of 3 to 10 at % Sn and 1 to 10 at % Sc, and the balance Ti and unavoidable impurities.
 2. An article according to claim 1, wherein said Ti-based alloy contains 4 to 10 at % Mo, 3 to 10 at % Sn, and the balance Ti and unavoidable impurities and has been subjected to an aging treatment at a temperature of 500° C. or more after a heat treatment at a β-transformation temperature or more.
 3. An article according to claim 1, wherein said Ti-based alloy contains 4 to 10 at % Mo, 1 to 10 at % Sc, and the balance Ti and unavoidable impurities and has been subjected to an aging treatment at a temperature of 150° C. or more after a heat treatment at a β-transformation temperature or more.
 4. An article according to claim 1, wherein said Ti-based shape memory alloy article is used at at least a human body temperature.
 5. An article according to claim 1, wherein said Ti-based shape memory alloy article has one of a plate shape, a tubular shape, and a linear shape including a bar shape.
 6. An article according to claim 1, wherein said Ti-based shape memory alloy article has one of a shape memory function and a superelasticity function.
 7. A medical material or a medical instrument comprising the article according to claim
 1. 8. A consumer article comprising the article according to claim 1 which forms part or the whole of one of a golf club, eye glasss, and a timepiece.
 9. An article according to claim 1, wherein said Ti-based shape memory alloy article is one of a heat exchanger, a heat insulator, and a vibration suppressor.
 10. An article of Ti-based shape memory alloy comprising a Ti-based alloy having a composition containing 15 to 30 at % Nb, 1 or more at % Sc, and the balance Ti and unavoidable impurities.
 11. An article according to claim 10, wherein said Ti-based alloy has been subjected to an aging treatment at a temperature of 500° C. or more after a heat treatment at a β-transformation temperature or more.
 12. An article according to claim 10, wherein said Ti-based shape memory alloy article is used at at least a human body temperature.
 13. An article according to claim 10, wherein said Ti-based shape memory alloy article has one of a plate shape, a tubular shape, and a linear shape including a bar shape.
 14. An article according to claim 10, wherein said Ti-based shape memory alloy article has one of a shape memory function and a superelasticity function.
 15. A medical material or a medical instrument comprising the article according to claim
 10. 16. A consumer article comprising the article according to claim 10 which forms part or the whole of one of a golf club, eye glasss, and a timepiece.
 17. An article according to claim 10, wherein said Ti-based shape memory alloy article is one of a heat exchanger, a heat insulator, and a vibration suppressor. 